In an attempt to reduce waste and pollution associated with traditional methods for disposing of products containing polyamide resins, including polyamide 6 (PA6) and polyamide 66 (PA66), efforts have been made to recycle the resins for use in other products. For example, one source of waste is from post-consumer carpet, carpet scrap, and carpet fibers, which can contain a significant amount of PA6 and PA66. Traditionally the carpet waste has been disposed by either dumping it in landfills or by incineration. However, neither option is environmentally friendly since carpet waste is not biodegradable and can emit noxious fumes if incinerated. Accordingly, alternative methods for dealing with the waste have been designed. See, e.g., Corina Mihut et al., Review: Recycling of Nylon From Carpet Waste, Polymer Engineering and Science, Vol. 41, No. 9, September 2001.
One method for dealing with waste containing PA6 and PA66 involves depolymerizing the resins into their constituent monomers (i.e., caprolactam, and hexamethylenediamine and adipic acid, respectively). For example, the method described in U.S. Pat. No. 5,681,952 uses superheated steam at high pressures to depolymerize PA6. Other depolymerization methods use ammonia, acids, and bases. In particular, U.S. Pat. No. 5,302,756 discloses depolymerizing PA66 and PA6 by using ammonia at high temperatures and pressures, and optionally a phosphate catalyst. See also U.S. Pat. No. 5,668,227 (describes a continuous depolymerization process for PA66 and PA6 using ammonia at high temperatures and pressure). U.S. Pat. No. 5,169,870 describes a method for depolymerizing PA6 using phosphoric acid and superheated steam between 230° C. and 320° C. Moreover, U.S. Pat. Nos. 5,294,707; 5,310,905; and 5,468,900 disclose depolymerization processes using various acids, while U.S. Pat. Nos. 5,233,037; 5,266,694; and 6,087,494 use various basic catalysts.
Yet depolymerization processes have numerous drawbacks. To begin, the processes have to be conducted at very high temperatures using potentially caustic or hazardous materials. Further, the starting materials used in the depolymerization processes must generally contain low amounts of impurities. While this may not be problematic for relatively pure starting materials, many sources of waste, including post-consumer carpet, contain high levels of impurities (e.g., incompatible carpet constituents, dirt, cleaning chemicals, pet urine, etc.). In order to remove or reduce the impurities, an additional separating or recycling step is required before the depolymerization process. However, additional processing steps not only add to the complexity of the overall recycling process, but also add to the cost of the end product.
In order to avoid some of the problems associated with depolymerization processes, alternatively, solvent-based extraction methods can be used to recycle PA6 and PA66 from waste. U.S. Pat. No. 5,840,773 discloses an extraction process using aliphatic alcohols, preferably methanol and ethanol, at high temperatures. And U.S. Pat. No. 5,898,063 discloses a solvent-based process using glycerol to dissolve PA6 at 155° C. and PA66 at 195° C.
Nevertheless, like depolymerization processes, solvent-based methods for recycling PA6 and PA66 also have their drawbacks. In particular, the processes can use potentially harmful solvents. And like depolymerization processes, solvent-based methods can be costly and time consuming because of the required processing conditions. Moreover, and even more concerning, are the problems associated with the solvent itself. For instance, solvents can degrade the PA6 and PA66 polymer chains, which will affect the physical properties of the recycled product. Solvents can also dissolve a high amount of impurities along with the polyamide resins, which will negatively affect the properties of the end product. In this respect, picking the proper solvent or mixture of solvents for any given type of waste, and running the process under the proper conditions, can be difficult and expensive.
To overcome the deficiencies and problems associated with both solvent-based and depolymerization recycling processes for waste containing PA6 and PA66, blending methods have been developed. The blending processes typically involve melt blending either the entire waste product or part of the waste product into a recycled resin. However, because waste products usually contain components that are incompatible with PA6 and/or PA66, such as polyolefins and other polymers, compatibilizers generally have to be added to form a homogeneous resin. Typical compatibilizers include grafted olefinic polymers and copolymers, such as maleic anhydride grafted or acrylic acid grafted polypropylene homo- and copolymers. Additionally, because the polyamides are not completely separated from the incompatible compounds, the overall polyamide purity of the resins is much lower.
While blending processes generally excludes the drawbacks associated with depolymerization and solvent-based methods, nonetheless, other problems exist. For instance, products produced from the recycled resins can demonstrate inferior mechanical properties, especially when the resins have a low polyamide purity. In order to address the problems associated with mechanical underperformance, various fillers can be added, including glass fillers, and alkali metal and alkaline earth metal-based fillers.
Nevertheless, even though known fillers can help address problems generally related to mechanical and strength properties, and known compatibilizers can aid with homogenization issues for melt blending processes, blends including recycled polyamides, such as recycled PA6, PA66, or mixtures thereof, still exhibit problems with delamination. For example, delamination can occur between blends having recycled PA6, PA66, or mixtures thereof, or between a blend having recycled PA6, PA66, or mixtures thereof, and another polymer resin. Accordingly, there remains a need in the art for polyamide compositions that demonstrate improved anti-delamination. There also remains a need in the art for a process for producing polyamide compositions that demonstrate improved anti-delamination.